This is straddling the line between completely untrue and highly misleading. For cash accounts, brokers cannot lend your shares without a written agreement allowing it. Although the standard agreement could include agreeing to a securities lending program, this is not common in retail brokerage contracts. Additionally, even if that clause was in your contract, they have to notify you whenever they lend your shares. That right to notification CANNOT be waived in the agreement. So if you have a cash account, no one is lending your shares without your knowledge, period.
Even for margin accounts, the same requirements hold for fully paid securities. So if you have a margin account, but you're not actually borrowing any money, no one is lending your shares without your knowledge.
The only case where your broker might lend your securities without your knowledge is when you have a margin account and you are actually borrowing money.
Just read my brokers agreement - they can indeed lend my shares without notifying me for any purpose.
They pay interest?
also - the interest rate is often negative : ie the lender PAYS THE BORROWER to take the shares. The borrower posts cash collateral that the lender gets to earn interest on...
it's a fun part of the market
If you agree to the broker agreement, you're implicitly giving consent.
Even worse is Naked Short Selling, a practise where short sellers don't borrow the shares in the first place. This artificially creates shares and dilutes the value of the stock being shorted. Although illegal since Reg SHO was introduced, it still goes on due to lack of enforcement and comically low fines - google "Reg SHO Violations".
Some public pension funds forbid the lending of the shares they own for shorting. The minimal interest gained by lending shares is not worth the downward pressure to the stock price created by the short which undermines the asset value.
https://en.wikipedia.org/wiki/Naked_short_selling
https://en.wikipedia.org/wiki/Failure_to_deliver